Recent experiments indicate that: (1) the ingestion of increased amounts of the trace mineral selenium during the developmental period of the teeth increases dental caries in man and experimental animals, and (2) selenium may reduce the beneficial effect of fluoride on dental caries. The broad objective of the research proposed in this application is to understand the biochemical mechanism of how selenium causes an increased susceptibility to dental caries. Utilizing radioactive isotopes of selenium, we plan to study: (1) fluoride-selenium interaction by measuring the deposition of each element in the hard and soft tissues, and (2) the mechanism of uptake of selenium into enamel, dentine and proteins of enamel and dentine when the element is administered as organic or inorganic selenium.